1. Field of the Invention
This invention relates to hearing aids and is more particularly directed to the automatic control of hearing aids to enhance the operation under presently difficult conditions encountered by a user as, for example, receiving communication from a source of sound located close to the user but surrounded by a multiplicity of other sources of sound, as in a crowded room with numerous conversations and extraneous "noise" of varying sound levels and occurring at varying distances from the user of a hearing aid.
2. Prior Art
The prior art with which this invention is concerned ranges from relatively simple hearing aid instruments, which may contain what have come to be known as agc, "compression" or automatic gain control features, and which may also require a user to physically manipulate a volume control so as to compensate for variations in the sound field in which a hearing aid instrument is operated or may, as is shown in U.S. Pat. No. 4,461,025 issued July 17, 1984, include complicated circuitry for completely eliminating the background "noise" components of a sound field that are typically characterized as "noise" in the sense that the user of a hearing aid instrument does not desire to receive, or perceive, such components of a signal.
Prior art hearing aid instruments have utilized compression circuits for reducing output distortion as the total input signal exceeds the maximum capability of a hearing aid instrument system and this is accomplished by utilizing a typical form of automatic gain control circuit to eliminate the accompanying distortion when an excessive input signal is encountered.
One such automatic gain control for this purpose is designed to effect a gain reduction during a relatively short time interval, that is known as the "attack time", to prevent discomfort or annoyance to the user of the hearing air instrument due to an excessive signal. On the other hand, when the excessive signal is no longer present or diminished, it may be desirable to have a short or a long release time, dependent upon the environmental conditions in which the hearing aid instrument is used, and the characteristics of the input signal. The fast attack time, typically less than ten milliseconds, has been determined to be reasonable whereas a typical release time may vary from ten milliseconds to much greater intervals of time. The aural perception of the hearing air user is influenced by the release time.